This invention relates to an apparatus for detecting a drop-out in a magnetic recording/reproducing apparatus, in which an audio signal is recorded.
Recently, in a general type of the magnetic recording/reproducing apparatus, that is, a video tape recorder (hereinafter, VTR), the video signal is recorded on a magnetic tape as oblique record tracks by rotary heads, and the audio signal is recorded on the edge of the magnetic tape as a longitudinal track by a stationary head. In the VTR, it is desired to record the video signal for a time period which is as long as possible in the magnetic tape, especially in the consumer type VTR. In order to increase the number of recording hours, it is necessary to make the recording density high. Accordingly, it is necessary to make the track width narrow and to make the tape feeding speed slow. As a result, the tape running velocity thereof has to be much slower than the velocity of a compact cassette recorder for an audio signal, that is, about 4.5 cm/s. This slow velocity does not effect the recording/reproducing of the video signal, because the rotary velocity of the rotary heads predominates over the tape running velocity in the relative velocity of the rotary heads to the magnetic tape. However, it effects the recording/reproducing of the audio signal by the stationary head so that it is difficult to maintain the necessary quality of the reproduced audio signal, for example, a reproducing frequency characteristic and a wow and flutter characteristic.
The recording of the audio signal together with the video signal has been investigated. In such a method of recording, only a small part of the broad band for the video signal is alotted to the audio signal to maintain the necessary quality mentioned above so that the high quality of the reproduced audio signal can be obtained. In one of the methods, as shown in FIGS. 1A and 1B, the audio signal (A) is frequency modulated, and the frequency modulated (hereinafter FM) audio signal is inserted between the band for an FM luminance signal (Y) and the low band for a down-converted chrominance signal (C1) (FIG. 1A), or inserted below the low band for a down-converted chrominance signal (C2) (FIG. 1B). Further, in another method in which the chrominance signal is time-multiplexed with the luminance signal, the FM audio signal is superimposed below the FM video signal. Hereinafter, these audio signal recording systems are called an FM audio superimposed system. In the FM audio superimposed system, it is necessary to cope with a drop-out. As record tracks are very narrow, defects of the magnetic tape or dust easily cause the drop-out, which raises a discontinuity of the FM signal which generates large amplitude noises, so that the quality of the reproduced audio signal becomes inferior.
Therefore, in general, in order to compensate for the drop-out, a generating period of the drop-out is detected by tracing the amplitude of the reproduced FM audio signal, and a level of the demodulated audio signal which occurs just before the generation of the drop-out is held during the generating period of the drop-out.
FIG. 2 shows a general block diagram of the audio signal recording/reproducing circuit in the FM audio superimposed system containing a compensating circuit for the drop-out. In FIG. 2 during a record mode, the audio signal supplied from an input terminal 1 is emphasized in a high band by a pre-emphasis circuit 2, and modulated by a frequency modulator. The FM audio signal, an unnecessary part of which is eliminated by a band pass filter (hereinafter BPF) 4, is adjusted by a level regulator 5. After that, the FM audio signal is added with the FM video signal supplied from an input terminal 7 by an adder 6 and recorded on the magnetic tape 9 by a magnetic head 8. In playback mode, the reproduced signal reproduced from the magnetic tape 9 by the magnetic head 8 is amplified by a pre-amplifier 10. One part thereof is supplied to a video signal reproducing circuit, which is not shown in FIG. 2, from an output terminal 11. Another part thereof is supplied to a BPF 12 which extracts only the FM audio signal. Some of the extracted FM audio signal is limitedly amplified by a limiter 13 with the rest thereof being detected by an amplitude detector 19.
The FM audio signal limitedly amplified by the limiter 13 is demodulated by a FM demodulator 14 and supplied to a hold circuit 16 through a low pass filter (hereinafter LPF) 15, which eliminates FM carrier components, etc. In order to compensate for the drop-out and the noise generated at the exchange point of the rotary heads, the hold circuit 16 samples the demodulated audio signal just before the noise generates and holds it during the noise generating period.
The hold circuit 16 is controlled by an output of an adder 22. The adder 22 adds an output of a drop-out detector 21 to a signal indicating the exchange points of the rotary heads supplied from an input terminal 23. The drop-out detector 21 is constructed by the amplitude detector 19 which detects an amplitude of the reproduced FM audio signal and a drop-out discriminator 20 which discriminates the drop-out generating period from the output of the amplitude detector 19. The demodulated audio signal compensated at the hold circut 16 is supplied through a de-emphasis circuit 17 to an output terminal 18 as a reproduced audio signal.
As mentioned above, the circuit shown in FIG. 2 can compensate for the noise raised by the drop-out, but has some problems, which are mainly due to the fact that the detection of the drop-out is made by directly tracing the amplitude of the reproduced FM audio signal. Namely, the drop-out is detected by discriminating whether the amplitude of the reproduced FM audio signal is large or not, but, in fact, the amplitude fluctuates due to factors as listed below, for example:
(1) A fluctuation in the recording level of the FM audio signal; PA0 (2) a fluctuation in the reproducing level according to a fluctuation of the recording/reproducing characteristic of the magnetic heads, for example, a gap length, a track width, etc.; PA0 (3) a fluctuation of a gain of the pre-amplifier; and PA0 (4) a fluctuation of the characteristic of the magnetic tape. PA0 (5) a fluctuation in the reproducing level according to a tracking error, which cannot be cancelled by the level adjustments. PA0 (1) a fluctuation of the gain of the pre-amplifier; and PA0 (2) a fluctuation of the noise of the pre-amplifier.
It is, therefore, at least necessary to adjust an input level of the amplitude detector 19 or a discriminating level of the drop-out discriminator 20 to compensate for such fluctuations. Further, in consideration of the compatability of each VTR, which is the most important factor, the following fluctuation must be considered:
In addition to the fluctuation of the level of the reproduced FM signal, there is also a fluctuation of the noise level. As impedance noises dominate in the fluctuation of the noise level, the following fluctuations have to be mainly considered:
The example for these fluctuation factors are listed in Table 1.
TABLE 1 ______________________________________ Factors Examples ______________________________________ Signal 1 Recording level .+-.2 dB 2 Magnetic head .+-.4 dB 3 Pre-amp. gain .+-.3.5 dB 4 Magnetic tape .+-.2 dB 5 Tracking error .+-.6 dB Total (3.delta.) .+-.8.5 dB Noise 1 Pre-amp. gain .+-.3.5 dB 2 Pre-amp. noise .+-.5 dB Total (3.delta.) .+-.6.1 dB ______________________________________ .delta.: a standard deviation
Apparently, from Table 1, the fluctuation of the reproduced FM signal is about .+-.8.5 dB, and the fluctuation of the noise level is about .+-.6.1 dB.
Generally, in the FM audio superimposed system, in order to reduce an interruption of the FM audio signal to the FM video signal, the recording level of the FM audio signal has to be lower than the recording level of the FM video signal, for example, about -25 dB in comparison with the latter. Therefore, a S/N ratio of the reproduced FM audio signal to the noise level does not become large. Assuming that the S/N ratio is about 20 dB, the S/N ratio varies as shown in FIG. 3A due to the fluctuations described above. Therefore, at the worst case, the S/N ratio becomes about 5.5 dB.
Now, it may be considered to use an auto gain control circuit (hereinafter, AGC for removing the fluctuations of the reproduced FM audio signal. However, the use of the AGC generates a new problem. That is, since the S/N ratio cannot be made large, the fluctuation of the level of an output noise due to the AGC causes mistakes in the drop-out detection, and as the AGC becomes to have a large gain and amplifies the noise at the portion in which the FM audio signal is not recorded, it becomes impossible to detect the drop-out.
There is a type of VTR which can vary the recording density, that is, the width of the recording tracks by changing the tape running velocity. In this type of VTR, the level of the reproduced FM audio signal varies in response to the variation of the width of the recording tracks. For example, in VTR which can double the recording density by setting the tape running velocity to be half of the normal velocity, the width of the recording tracks becomes half of the normal one, so that the level of the reproduced FM audio signal goes down about 6 dB. Accordingly, the S/N ratio of the reproduced FM audio signal to the noise level, which include fluctuations of the reproduced FM audio signal and fluctuations of the noise level, is represented in FIG. 3B.
Apparently, from FIG. 3B, the S/N ratio becomes -1 dB at the worst case, so that it would be almost impossible to detect the drop-out by the method of detecting the amplitude of the reproduced FM audio signal. Further, this method cannot detect a drop-in due to the noise which jumps into the reproduced FM signal.
As described above, there are some problems in the method for detecting the drop-out or the drop-in by detecting the amplitude of the reproduced FM audio signal.